Closed-loop control in a linear motion control system does not require adjustments to be made in order to achieve a desired output (or maintain a constant output) despite varying conditions. Typically, closed-loop control is performed by a microcontroller executing firmware that compares the system's output signal with a desired command to determine the drive for the system. Thus, drive input is adjusted until the output signal matches the desired command.
In some linear positioning applications, the microcontroller uses output signal data from a number of different components, including a position sensor. For example, in small linear motor applications, a magnetic field sensor such as a Hall-Effect sensor may be used to sense motor position, which is then used by the microcontroller to supply a drive current to the motor. In linear motion control applications without position sensor feedback, a co-processor (such as digital signal processor) may be used to derive the position information, for example, by characterizing linear displacement as a function of drive current.
Closed-loop control of this kind is not without problems, however. To ensure loop stability, it is often necessary to operate the system very slowly. Such operation results in long response times to changes in the motor's position. Also, precise linear motion control may be difficult to achieve due to hysteresis in the motor's mechanical movement.